This invention relates to a frequency division multi-access communication system for transmitting a carrier modulated with an information signal and, more particularly, to a radio reception unit employing a spread spectrum modulation system.
Two methods have been generally used for a communication system for transmitting information through spread spectrum modulation with bandwidth wider than that of the transmission information. One is a frequency hopping method for changing the carrier responding to changes in plural frequencies upon transmission. The other is a direct spread method for modulating the carrier by using a spread code with its bandwidth wider than that of the information signal. A Japanese Patent Application Laid-Open No. 36696 (1988) titled "Reception Unit" discloses a radio reception unit employing the aforementioned communication system.
FIG. 4 is a block diagram of a representative construction of a conventional radio reception unit.
Referring to FIG. 4, the radio reception unit comprises 25 an antenna 1 for receiving a transmission wave, a broad-band band pass filter (BPF) 2 through which the received signal from the antenna 1 passes, a high frequency by-pass selector 3 for selecting an output or non-output by switching ON (energize) or OFF (deenergize) the received signal from the antenna 1 and a narrow-band band pass filter (BPF) 4 (4a, 4b, . . . , 4n) each of which is set to have a different center frequency.
The reception unit further comprises a narrow-band BPF selector 5 (5a, 5b) for selecting one of the narrow-band BPF 4 (4a, . . . , 4n) through switching input/output terminals thereof, a control section 6 for controlling the selection of the high-frequency by-pass selector 3 and the narrow-band BPF selector 5 based on a received signal sent from the broad-band BPF 2, a demodulation section 7 for demodulating the received signal sent from either the broad-band BPF 2 or the narrow-band BPF 4 and an output terminal (loud speaker) 8 for outputting a demodulated signal sent from the demodulation section 7.
An operation of this prior art is described.
A received signal from the antenna 1 is input to moving contacts of the high frequency by-pass selector 3 and the narrow-band BPF selector 5. The high frequency by-pass selector 3 is set to ON (energized) through the control section 6 at an initial reception stage when it is not synchronized. The received signal from the high frequency by-pass selector 3 is input to the control section 6 and the demodulation section 7 through the broad-band BPF 2.
Being synchronized through the control section 6, the high frequency by-pass selector 3 is set to OFF (deenergized). The narrow-band BPF 5 concurrently selects one narrow-band BPF 4 so as to match with the received frequency through the control section 6. The demodulation section 7 receives one output of either the broad-band BPF 2 or the narrow-band BPF 4 and sends the demodulated output to the output terminal (loud speaker) 8.
In the conventional radio reception unit, the BPF with its bandwidth matching the received frequency is selected among those with different bandwidth for transmitting information with plural frequency bands.
The reception units of the similar type have been proposed in Publications of Japanese Patent Application Laid-Open Nos. 36696 (1988) titled "Reception Unit"; 54033 (1988) titled "Spread Spectrum Modulation/Demodulation Unit"; 32735 (1989) titled "Code Division Multi-Access Communication Unit"; 92029 (1991) titled "Spread Spectrum Reception Unit"; 174831 (1991) titled "Spread Spectrum Reception Unit"; and 91528 (1992) titled "Spread Spectrum Reception Unit".
In the above reception unit, a suitable BPF is selected among those with different bandwidth and the interfering wave is removed for quality reception. Therefore it is difficult for such a reception unit to reduce the circuitry size as well as circuit integration. Furthermore the reception characteristic is likely to vary with the received carrier frequency.
FIG. 5(a), (b) and (c) are graphical views showing high-speed and low-speed transmissions of video data with the spread spectrum communication system.
Recently the spread spectrum communication system employing the direct spread method has been increasingly used. In the video data transmission as shown in FIG. 5(a), an audio signal of low-speed data is transmitted. In the video data transmission as shown in FIG. 5(b), a video signal of high-speed data is transmitted. In case an audio signal of low-speed data has a small amount of transmission information, the spread bandwidth is decreased and plural carriers are frequency division multi-accessed. The assigned frequency band is divided into plural frequency bands (SC1, SC2, SC3 and SC4) for transmitting plural kinds of information. Therefore the efficiency for using the radio channel (frequency) can be improved.
While in case the high speed video data are transmitted, the spread bandwidth is broadened to accommodate a large amount of the transmission information. Therefore the whole assigned frequency band is used with one carrier without being divided. As FIG. 5(c) shows, the transmission superimposing the low-speed data and high-speed data is executed. In this case, data are normally transmitted through the narrow-band spreading. The high-speed data are transmitted through the broad-band spreading switched by a user.
It has been proposed to provide a transmission system for improving the efficiency for using the radio channel (frequency) through frequency division multi-accessing a combination of plural spread bandwidth and the carrier in compliance with the transmission information size. This transmission system requires many filters adapted to the reception frequency and reception bandwidth. Therefore it is difficult for such a system to have down-sizing and circuit integration.
Further the selector for selecting the I/O terminal of plural narrow-band filters is required to have stable frequency characteristic (frequency passing characteristic). The frequency characteristic is likely to be unstable accompanied with broadening of the frequency bandwidth. That is, it becomes difficult to have plural narrow-band filters having required stable frequency characteristics.
It is also difficult to set those narrow-band filters to have the same stable frequency characteristics and yet have the respective center frequencies slightly different with each other. That is, the reception characteristic is likely to vary with the reception carrier frequency.